[libc] Add a linux Thread class in __support/threads.

This change is essentially a mechanical change which moves the thread
creation and join implementations from src/threads/linux to
src/__support/threads/linux/thread.h. The idea being that, in future, a
pthread implementation can reuse the common thread implementations in
src/__support/threads.

Reviewed By: lntue

Differential Revision: https://reviews.llvm.org/D123287

9 files changed, 83 insertions, 264 deletions

diff --git a/libc/src/threads/CMakeLists.txt b/libc/src/threads/CMakeLists.txt
index a02fc161ecc5..bd63a53061f3 100644
--- a/libc/src/threads/CMakeLists.txt
+++ b/libc/src/threads/CMakeLists.txt
@@ -11,16 +11,28 @@ add_entrypoint_object(
 
 add_entrypoint_object(
   thrd_create
-  ALIAS
+  SRCS
+    thrd_create.cpp
+  HDRS
+    thrd_create.h
   DEPENDS
-    .${LIBC_TARGET_OS}.thrd_create
+    libc.src.__support.threads.thread
+    libc.include.threads
+  COMPILE_OPTIONS
+    -O3
+    -fno-omit-frame-pointer # This allows us to sniff out the thread args from
+                            # the new thread's stack reliably.
 )
 
 add_entrypoint_object(
   thrd_join
-  ALIAS
+  SRCS
+    thrd_join.cpp
+  HDRS
+    thrd_join.h
   DEPENDS
-    .${LIBC_TARGET_OS}.thrd_join
+    libc.include.threads
+    libc.src.__support.threads.thread
 )
 
 add_entrypoint_object(
diff --git a/libc/src/threads/linux/CMakeLists.txt b/libc/src/threads/linux/CMakeLists.txt
index 0abdf5d23363..339c2b5dc3d5 100644
--- a/libc/src/threads/linux/CMakeLists.txt
+++ b/libc/src/threads/linux/CMakeLists.txt
@@ -17,48 +17,13 @@ add_header_library(
   HDRS
     CndVar.h
     Futex.h
-    Thread.h
   DEPENDS
     libc.include.sys_syscall
     libc.include.threads
     libc.src.__support.CPP.atomic
     libc.src.__support.OSUtil.osutil
     libc.src.__support.threads.mutex
-)
-
-add_entrypoint_object(
-  thrd_create
-  SRCS
-    thrd_create.cpp
-  HDRS
-    ../thrd_create.h
-  DEPENDS
-    .threads_utils
-    libc.include.errno
-    libc.include.sys_mman
-    libc.include.sys_syscall
-    libc.include.threads
-    libc.src.__support.common
-    libc.src.__support.OSUtil.osutil
-  COMPILE_OPTIONS
-    -O3
-    -fno-omit-frame-pointer # This allows us to sniff out the thread args from
-                            # the new thread's stack reliably.
-)
-
-add_entrypoint_object(
-  thrd_join
-  SRCS
-    thrd_join.cpp
-  HDRS
-    ../thrd_join.h
-  DEPENDS
-    .threads_utils
-    libc.include.sys_syscall
-    libc.include.threads
-    libc.src.__support.CPP.atomic
-    libc.src.__support.common
-    libc.src.__support.OSUtil.osutil
+    libc.src.__support.threads.linux.futex_word_type
 )
 
 add_entrypoint_object(
diff --git a/libc/src/threads/linux/CndVar.h b/libc/src/threads/linux/CndVar.h
index 2b914d480c76..009cb9f06c3f 100644
--- a/libc/src/threads/linux/CndVar.h
+++ b/libc/src/threads/linux/CndVar.h
@@ -13,6 +13,7 @@
 #include "include/threads.h"     // For values like thrd_success etc.
 #include "src/__support/CPP/atomic.h"
 #include "src/__support/OSUtil/syscall.h" // For syscall functions.
+#include "src/__support/threads/linux/futex_word.h"
 #include "src/__support/threads/mutex.h"
 
 #include <linux/futex.h> // For futex operations.
@@ -106,7 +107,7 @@ struct CndVar {
     if (waitq_front == nullptr)
       waitq_back = nullptr;
 
-    qmtx.futex_word = Mutex::FutexWordType(Mutex::LockState::Free);
+    qmtx.futex_word = FutexWordType(Mutex::LockState::Free);
 
     __llvm_libc::syscall(
         SYS_futex, &qmtx.futex_word.val, FUTEX_WAKE_OP, 1, 1,
diff --git a/libc/src/threads/linux/Thread.h b/libc/src/threads/linux/Thread.h
deleted file mode 100644
index de58ae4c64c6..000000000000
--- a/libc/src/threads/linux/Thread.h
+++ /dev/null
@@ -1,23 +0,0 @@
-//===-- Linux specific definitions for threads implementations. --*- C++ -*===//
-//
-// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
-// See https://llvm.org/LICENSE.txt for license information.
-// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
-//
-//===----------------------------------------------------------------------===//
-
-#ifndef LLVM_LIBC_SRC_THREADS_LINUX_THREAD_UTILS_H
-#define LLVM_LIBC_SRC_THREADS_LINUX_THREAD_UTILS_H
-
-#include <stdint.h>
-
-namespace __llvm_libc {
-
-struct ThreadParams {
-  static constexpr uintptr_t DEFAULT_STACK_SIZE = 1 << 16; // 64 KB
-  static constexpr uint32_t CLEAR_TID_VALUE = 0xABCD1234;
-};
-
-} // namespace __llvm_libc
-
-#endif // LLVM_LIBC_SRC_THREADS_LINUX_THREAD_UTILS_H
diff --git a/libc/src/threads/linux/thrd_create.cpp b/libc/src/threads/linux/thrd_create.cpp
deleted file mode 100644
index 5f25ded38bd3..000000000000
--- a/libc/src/threads/linux/thrd_create.cpp
+++ /dev/null
@@ -1,153 +0,0 @@
-//===-- Linux implementation of the thrd_create function ------------------===//
-//
-// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
-// See https://llvm.org/LICENSE.txt for license information.
-// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
-//
-//===----------------------------------------------------------------------===//
-
-#include "Futex.h"
-
-#include "src/__support/OSUtil/syscall.h" // For syscall function.
-#include "src/__support/architectures.h"
-#include "src/__support/common.h"
-#include "src/threads/linux/Thread.h"
-#include "src/threads/thrd_create.h"
-
-#include <errno.h>       // For E* error values.
-#include <linux/sched.h> // For CLONE_* flags.
-#include <stdint.h>
-#include <sys/mman.h>    // For PROT_* and MAP_* definitions.
-#include <sys/syscall.h> // For syscall numbers.
-#include <threads.h>     // For thrd_* type definitions.
-
-#ifdef SYS_mmap2
-constexpr long MMAP_SYSCALL_NUMBER = SYS_mmap2;
-#elif SYS_mmap
-constexpr long MMAP_SYSCALL_NUMBER = SYS_mmap;
-#else
-#error "SYS_mmap or SYS_mmap2 not available on the target platform"
-#endif
-
-namespace __llvm_libc {
-
-// We align the start args to 16-byte boundary as we adjust the allocated
-// stack memory with its size. We want the adjusted address to be at a
-// 16-byte boundary to satisfy the x86_64 and aarch64 ABI requirements.
-// If different architecture in future requires higher alignment, then we
-// can add a platform specific alignment spec.
-struct alignas(16) StartArgs {
-  thrd_t *thread;
-  thrd_start_t func;
-  void *arg;
-};
-
-__attribute__((always_inline)) inline uintptr_t get_start_args_addr() {
-  // NOTE: For __builtin_frame_address to work reliably across compilers,
-  // architectures and various optimization levels, the TU including this file
-  // should be compiled with -fno-omit-frame-pointer.
-  return reinterpret_cast<uintptr_t>(__builtin_frame_address(0))
-         // The x86_64 call instruction pushes resume address on to the stack.
-         // Next, The x86_64 SysV ABI requires that the frame pointer be pushed
-         // on to the stack. Similarly on aarch64, previous frame pointer and
-         // the value of the link register are pushed on to the stack. So, in
-         // both these cases, we have to step past two 64-bit values to get
-         // to the start args.
-         + sizeof(uintptr_t) * 2;
-}
-
-static __attribute__((noinline)) void start_thread() {
-  StartArgs *start_args = reinterpret_cast<StartArgs *>(get_start_args_addr());
-  __llvm_libc::syscall(SYS_exit, start_args->thread->__retval =
-                                     start_args->func(start_args->arg));
-}
-
-LLVM_LIBC_FUNCTION(int, thrd_create,
-                   (thrd_t * thread, thrd_start_t func, void *arg)) {
-  unsigned clone_flags =
-      CLONE_VM        // Share the memory space with the parent.
-      | CLONE_FS      // Share the file system with the parent.
-      | CLONE_FILES   // Share the files with the parent.
-      | CLONE_SIGHAND // Share the signal handlers with the parent.
-      | CLONE_THREAD  // Same thread group as the parent.
-      | CLONE_SYSVSEM // Share a single list of System V semaphore adjustment
-                      // values
-      | CLONE_PARENT_SETTID   // Set child thread ID in |ptid| of the parent.
-      | CLONE_CHILD_CLEARTID; // Let the kernel clear the tid address and futex
-                              // wake the joining thread.
-  // TODO: Add the CLONE_SETTLS flag and setup the TLS area correctly when
-  // making the clone syscall.
-
-  // Allocate thread stack.
-  long mmap_result =
-      __llvm_libc::syscall(MMAP_SYSCALL_NUMBER,
-                           0, // No special address
-                           ThreadParams::DEFAULT_STACK_SIZE,
-                           PROT_READ | PROT_WRITE,      // Read and write stack
-                           MAP_ANONYMOUS | MAP_PRIVATE, // Process private
-                           -1, // Not backed by any file
-                           0   // No offset
-      );
-  if (mmap_result < 0 && (uintptr_t(mmap_result) >=
-                          UINTPTR_MAX - ThreadParams::DEFAULT_STACK_SIZE)) {
-    return -mmap_result == ENOMEM ? thrd_nomem : thrd_error;
-  }
-  void *stack = reinterpret_cast<void *>(mmap_result);
-
-  thread->__stack = stack;
-  thread->__stack_size = ThreadParams::DEFAULT_STACK_SIZE;
-  thread->__retval = -1;
-  FutexWordType *clear_tid_address = &thread->__clear_tid.__word;
-  *clear_tid_address = ThreadParams::CLEAR_TID_VALUE;
-
-  // When the new thread is spawned by the kernel, the new thread gets the
-  // stack we pass to the clone syscall. However, this stack is empty and does
-  // not have any local vars present in this function. Hence, one cannot
-  // pass arguments to the thread start function, or use any local vars from
-  // here. So, we pack them into the new stack from where the thread can sniff
-  // them out.
-  uintptr_t adjusted_stack = reinterpret_cast<uintptr_t>(stack) +
-                             ThreadParams::DEFAULT_STACK_SIZE -
-                             sizeof(StartArgs);
-  StartArgs *start_args = reinterpret_cast<StartArgs *>(adjusted_stack);
-  start_args->thread = thread;
-  start_args->func = func;
-  start_args->arg = arg;
-
-  // The clone syscall takes arguments in an architecture specific order.
-  // Also, we want the result of the syscall to be in a register as the child
-  // thread gets a completely different stack after it is created. The stack
-  // variables from this function will not be availalbe to the child thread.
-#ifdef LLVM_LIBC_ARCH_X86_64
-  long register clone_result asm("rax");
-  clone_result = __llvm_libc::syscall(
-      SYS_clone, clone_flags, adjusted_stack,
-      &thread->__tid,    // The address where the child tid is written
-      clear_tid_address, // The futex where the child thread status is signalled
-      0                  // Set TLS to null for now.
-  );
-#elif defined(LLVM_LIBC_ARCH_AARCH64)
-  long register clone_result asm("x0");
-  clone_result = __llvm_libc::syscall(
-      SYS_clone, clone_flags, adjusted_stack,
-      &thread->__tid,   // The address where the child tid is written
-      0,                // Set TLS to null for now.
-      clear_tid_address // The futex where the child thread status is signalled
-  );
-#else
-#error "Unsupported architecture for the clone syscall."
-#endif
-
-  if (clone_result == 0) {
-    start_thread();
-  } else if (clone_result < 0) {
-    __llvm_libc::syscall(SYS_munmap, mmap_result,
-                         ThreadParams::DEFAULT_STACK_SIZE);
-    int error_val = -clone_result;
-    return error_val == ENOMEM ? thrd_nomem : thrd_error;
-  }
-
-  return thrd_success;
-}
-
-} // namespace __llvm_libc
diff --git a/libc/src/threads/linux/thrd_join.cpp b/libc/src/threads/linux/thrd_join.cpp
deleted file mode 100644
index 30275ff73754..000000000000
--- a/libc/src/threads/linux/thrd_join.cpp
+++ /dev/null
@@ -1,46 +0,0 @@
-//===-- Linux implementation of the thrd_join function --------------------===//
-//
-// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
-// See https://llvm.org/LICENSE.txt for license information.
-// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
-//
-//===----------------------------------------------------------------------===//
-
-#include "Futex.h"
-
-#include "src/__support/CPP/atomic.h"
-#include "src/__support/OSUtil/syscall.h" // For syscall function.
-#include "src/__support/common.h"
-#include "src/threads/linux/Thread.h"
-#include "src/threads/thrd_join.h"
-
-#include <linux/futex.h> // For futex operations.
-#include <sys/syscall.h> // For syscall numbers.
-#include <threads.h>     // For thrd_* type definitions.
-
-namespace __llvm_libc {
-
-LLVM_LIBC_FUNCTION(int, thrd_join, (thrd_t * thread, int *retval)) {
-  auto *clear_tid_address =
-      reinterpret_cast<cpp::Atomic<FutexWordType> *>(&thread->__clear_tid);
-
-  // The kernel should set the value at the clear tid address to zero.
-  // If not, it is a spurious wake and we should continue to wait on
-  // the futex.
-  while (clear_tid_address->load() != 0) {
-    // We cannot do a FUTEX_WAIT_PRIVATE here as the kernel does a
-    // FUTEX_WAKE and not a FUTEX_WAKE_PRIVATE.
-    __llvm_libc::syscall(SYS_futex, &clear_tid_address->val, FUTEX_WAIT,
-                         ThreadParams::CLEAR_TID_VALUE, nullptr);
-  }
-
-  *retval = thread->__retval;
-
-  if (__llvm_libc::syscall(SYS_munmap, reinterpret_cast<long>(thread->__stack),
-                           thread->__stack_size) == -1)
-    return thrd_error;
-
-  return thrd_success;
-}
-
-} // namespace __llvm_libc
diff --git a/libc/src/threads/thrd_create.cpp b/libc/src/threads/thrd_create.cpp
new file mode 100644
index 000000000000..91b36a71f823
--- /dev/null
+++ b/libc/src/threads/thrd_create.cpp
@@ -0,0 +1,33 @@
+//===-- Linux implementation of the thrd_create function ------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "src/threads/thrd_create.h"
+#include "src/__support/common.h"
+#include "src/__support/threads/thread.h"
+
+#include <errno.h>
+#include <threads.h> // For thrd_* type definitions.
+
+namespace __llvm_libc {
+
+static_assert(sizeof(thrd_t) == sizeof(__llvm_libc::Thread<int>),
+              "Mismatch between thrd_t and internal Thread<int>.");
+
+LLVM_LIBC_FUNCTION(int, thrd_create,
+                   (thrd_t * th, thrd_start_t func, void *arg)) {
+  auto *thread = reinterpret_cast<__llvm_libc::Thread<int> *>(th);
+  int result = thread->run(func, arg, nullptr, 0);
+  if (result == 0)
+    return thrd_success;
+  else if (result == ENOMEM)
+    return thrd_nomem;
+  else
+    return thrd_error;
+}
+
+} // namespace __llvm_libc
diff --git a/libc/src/threads/thrd_create.h b/libc/src/threads/thrd_create.h
index d2bb7dfad41c..9ed19c48c0a4 100644
--- a/libc/src/threads/thrd_create.h
+++ b/libc/src/threads/thrd_create.h
@@ -9,7 +9,7 @@
 #ifndef LLVM_LIBC_SRC_THREADS_THRD_CREATE_H
 #define LLVM_LIBC_SRC_THREADS_THRD_CREATE_H
 
-#include "include/threads.h"
+#include <threads.h>
 
 namespace __llvm_libc {
 
diff --git a/libc/src/threads/thrd_join.cpp b/libc/src/threads/thrd_join.cpp
new file mode 100644
index 000000000000..20096a7be1bf
--- /dev/null
+++ b/libc/src/threads/thrd_join.cpp
@@ -0,0 +1,30 @@
+//===-- Linux implementation of the thrd_join function --------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "src/threads/thrd_join.h"
+#include "src/__support/common.h"
+#include "src/__support/threads/thread.h"
+
+#include <threads.h> // For thrd_* type definitions.
+
+namespace __llvm_libc {
+
+static_assert(sizeof(thrd_t) == sizeof(__llvm_libc::Thread<int>),
+              "Mismatch between thrd_t and internal Thread<int>.");
+
+LLVM_LIBC_FUNCTION(int, thrd_join, (thrd_t * th, int *retval)) {
+  auto *thread = reinterpret_cast<Thread<int> *>(th);
+  int result = thread->join();
+  if (result == 0) {
+    *retval = thread->return_value();
+    return thrd_success;
+  }
+  return thrd_error;
+}
+
+} // namespace __llvm_libc